1. Field of the Invention
The present invention relates generally to electric machines or motor/generators and, more specifically, to permanent magnet, axial field electric machines.
2. Description of the Related Art
An electric motor/generator, referred to in the art as an electric machine, is a device that converts electrical energy to mechanical energy and/or mechanical energy to electrical energy. Since electric machines appear more commonly as motors, the ensuing discussion often assumes that electric energy is being converted to mechanical energy. However, those knowledgeable in the art recognize that the description below applies equally well to both motors and generators.
Electric machines generally operate based on Faraday""s law,. which can be written as e=BLv, and the Lorentz force equation, which is often written as F=BLi. In electric machines that utilize rotational motion, these equations can be written as e=k1BLxcexa9 and T=k2BLi respectively. Faraday""s law describes the speed voltage or back EMF (electromotive force), e, that appears across motor conductors due to the geometrically orthogonal interaction of a magnetic field having flux density B with conductors of length L traveling at a rotational speed xcexa9. The Lorentz force equation describes the torque T generated by the geometrically orthogonal interaction of a magnetic field having flux density, B, with conductors of length L carrying current i. The coefficients k1 and k2 are constants that are a function of motor geometry, material properties, and design parameters.
A variety of electric machine types exist in the art based on how they generate the magnetic field and on how they control the flow of electrical energy in the conductors exposed to the magnetic field. The present invention pertains to electric machines where the magnetic field B is primarily generated by permanent magnets affixed to the rotating assembly, or rotor of the machine; whereas the conductors are affixed to the stationary assembly, or stator of the machine and electronic circuitry is used to control the flow of electrical energy. In the art this type of machine is commonly called a brushless DC motor or a brushless permanent magnet machine. In addition, such electric machines can be modified to use induction to generate the magnetic field. In this case the machine is commonly called an induction motor.
Electric machines that produce rotational motion are classified as either radial field or axial field. Radial field machines have a radially directed magnetic field interacting with axially directed conductors, leading to rotational motion. On the other hand, axial field machines have an axially directed magnetic field interacting with radially directed conductors, leading to rotational motion. Of these two machine topologies, the axial field machine appears much less often. In the art, axial field machines are most often found in applications where: (i) there is insufficient axial length to accommodate a radial field machine, (ii) relatively little torque is needed, and (iii) motor energy conversion efficiency is not a primary concern. The reasons why axial field machines generally appear less often than radial field machines include: (a) more familiarity with radial field machines, (b) the desire to minimize cost by reusing existing radial field machine tooling, and (c) the lack of market incentive to address manufacturing issues unique to axial field machines.
In terms of quantity produced, the spindle motor in computer floppy disk drives is the most commonly appearing axial field electric machine. In this application minimizing cost is the most critical design goal. As a result, this motor does not utilize materials, design steps, or construction techniques that lead to high efficiency over a broad range of speeds, high motor constant, or high power density. The floppy disk spindle motor uses an axial field topology solely because there is insufficient axial space available inside the floppy disk housing to use a radial field motor. This motor is typically manufactured with one rotor element and one stator element, with the stator element being constructed from a steel-backed printed circuit board upon which the stator windings and motor electric drive circuitry are connected.
The present invention discloses design aspects for axial field machines that offer greater performance than common axial field machines and performance that meets, exceeds, or is competitive with radial field machines. Performance in this case includes the measures of: (i) energy conversion efficiency; (ii) motor constant, (iii) gravimetric power density, (iv) volumetric power density, (v) manufacturing cost, and (vi) construction flexibility due to modular construction.
Energy conversion efficiency describes how well an electric machine converts energy. For a motor, efficiency can be written as
xcex7=(Power Out)/(Power In)=(Txcexa9)/(Txcexa9+Pr+Pc+Pm)xe2x80x83xe2x80x83(Eq. 1)
where T is torque, xcexa9 is rotational speed, Pr is resistive loss i.e., the so called I2R loss, which represents power converted to heat by the resistance of the current carrying conductors in the motor, Pc is the core loss, which represents power converted to heat due to hysteresis and eddy current losses in the conductive and magnetic materials used in the motor, and Pm is the mechanical loss, which includes bearing loss, windage, etc. Core and mechanical losses generally increase with the square of speed, so efficiency typically increases from zero at zero speed, to some peak value at some rated speed, then decreases beyond that rated speed. For constant speed applications, achieving high peak efficiency at a constant rated speed is all that is important. For variable speed applications, however, it is important to maximize the range of speeds over which maximum efficiency can be achieved. As defined in Eq. 1, efficiency is unitless and is often expressed as a percentage, where 100% efficiency reflects the ideal electric machine.
Referring to FIG. 30, a graph is presented showing the efficiency of a typical electric machine known in the art at various speeds and torque. The operation of the electric machine is bounded by a peak speed, a peak torque, and a maximum power output. In this example, the electric machine has a peak efficiency of 90% at a particular operating point (i.e., at a particular rated speed and torque). At other operating points, however, the efficiency drops off precipitously as indicated by the contours of constant efficiency. In a traction application, for example, when the electric machine is operated at different operating points on the graph, the average efficiency will be much lower than peak efficiency.
In servomotor applications where a motor does not turn continuously but rather starts and stops frequently, efficiency is not a good measure of motor performance because efficiency is zero at zero speed, i.e., xcexa9=0. Under these conditions, the ability to produce torque with minimum losses is important. In the art the term motor constant describes the motor characteristic. Motor constant can be written and simplified as                               K          m                =                              T                                          P                r                                              =                                                                      K                  T                                ⁢                I                                                                                  I                    2                                    ⁢                  R                                                      =                                          K                T                                            R                                                                        (                  Eq          .                      xe2x80x83                    ⁢          2                )            
where KT is the motor torque constant, I is the net motor current, and R is the net motor resistance. Core loss and mechanical loss are not included in the motor constant because these losses are zero at zero speed. The square root of Pr is used in Eq. 2 because it makes the motor constant independent of current, which makes it independent of any motor load and makes it easier to compare the performance of different motors.
Based on Eq. 1 and Eq. 2, it is clear that a motor exhibiting high efficiency will generally exhibit a high motor constant. Likewise, if a motor exhibits minimal core loss and mechanical loss, a motor having a high motor constant will also exhibit high efficiency.
Gravimetric and volumetric power density are defined as the ratio of output power, e.g., Txcexa9 for a motor, to the mass and volume of the machine, respectively. As such, gravimetric power density is often specified in terms of watts per pound, horsepower per pound, or kilowatts per kilogram. Likewise, volumetric power density is often specified in terms of watts per cubic inch or kilowatts per cubic meter. In most cases, there is a high degree of correlation between these two measures of power density. That is, given that electric machines are generally constructed from the same types of materials, their mass is directly proportional to their volume, thus a motor having a high gravimetric power density, will also exhibit a high volumetric power density. Given this correlation, it is common to use the term power density to mean either gravimetric or volumetric power density or both. In any case, since output power is the product of torque and speed, power density increases linearly with speed to the point where it is no longer possible to maintain torque production, at which point power density decreases. In addition, given that torque is generally proportional to current as shown in Eq. 2, the ability to produce torque is only limited by the ability to remove the heat created by the resulting I2R loss Pr and the speed dependent losses Pc and Pm, which decrease efficiency. As a result, power density is generally proportional to efficiency because more power can be safely produced in a more efficient motor. For example, a highly efficient motor generates less heat for a given torque output than a less efficient motor, which in turn implies that the more highly efficient motor can generate more torque and therefore have higher power density, while generating the same amount of heat as the less efficient motor.
In the art, electric machines of varying outputs generally require significant unique tooling for each voltage and torque level. For a given diameter it is typical to specify a number of rotor and stator lengths, with similar but different parts and tooling required for each rotor and stator. For example, in a brushless DC motor each stator may be made from the same stator laminations stacked to various lengths, but the windings are unique for every length as well as for every voltage level at any fixed length. As a result, additional cost is incurred in traditional motors due to the additional capital expense and inventory required to support a family of motors at a given diameter.
In view of the above, there is a need for an improved axial field electric machine that provides a high efficiency over a wide variety of speeds and torque and a high gravimetric and volumetric power density over a wide range of speeds and torque. There is also a need for an improved axial field electric machine that allows for easy modification of the rotor and/or stator to increase or decrease the power output of the electric machine.
These and other needs are satisfied by the axial field electric machine of the present invention. Based on the above discussion, the present invention discloses design aspects for an axial field electric machine that maximize efficiency, motor constant, power density, as well as offer the benefits of modular construction, and the potential for reduced cost. Efficiency and motor constant are maximized by maximizing the production of torque while incurring minimal losses. In particular, one aspect of the invention eliminates all ferromagnetic material that incurs core loss, thereby essentially eliminating Pc from Eq. 1, above (although eddy current losses in the conductors must be considered). Doing so increases peak efficiency, broadens the range of speeds over which efficiency is high, and increases power density by eliminating the high mass associated with the added stationary ferromagnetic material. In addition, other aspects of the invention minimizes Pr, which maximize motor constant and maximizes the peak efficiency. Power density is maximized further according to an embodiment of the present invention by optimum selection of the amount of permanent magnet material relative to stator volume. Modular construction allows a whole family of motors at varying power levels to be constructed by stacking sets of identical rotor components and stator components axially within the same motor. Since each rotor and stator is identical, no duplication of capital cost is incurred to produce a whole family of motors. In addition, other aspects of the invention make it possible to select a variety of voltage levels by simply changing the way individual stators are connected, thereby minimizing the inventory required to support a whole family of motors.